Depends entirely on how much information you want to put on it. Newsletter, of course, on 8 1/2 x 11. Oversized postcard if you have more than one message to convey, or if you relly want it to stand out. Smaller postcards can get hidden behind other mail, oversized are harder to shuffle around. Regular postcard for 'reminder' type mailings. I use these now on the same color card every time just to put my name in my customers' heads every now and then. Postage costs might help you make a decision as well!